1. Field of the Invention
The present invention relates to a zoom lens system and an electronic imaging device that uses such a zoom lens system, for use in a digital camera, etc.
2. Description of Related Art
In a retractable zoom lens system which retracts the lens groups thereof by reducing the distances (spaces) therebetween during a lens-retracting operation and has a zoom ratio of approximately 3, it is common to utilize three lens groups of a negative-lead type zoom lens system in order to achieve further miniaturization thereof, as disclosed in, e.g., Japanese Unexamined Patent Publication Nos. 2002-48975, 2005-37727, 2006-276897 and 2007-286577.
Furthermore, zoom lens systems which use many glass aspherical lens elements in order to achieve a zoom ratio of approximately 3.6 and a wide angle-of-view at the wide extremity (short focal length extremity) are known in the art, as disclosed in, e.g., in Japanese Unexamined Patent Publication Nos. 2005-099091, 2005-084647, 2005-084648 and 2005-084649.
Moreover, it is common to utilize a zoom lens system that has four lens groups of a positive-lead type zoom lens system having a zoom ratio exceeding 4.
In the zoom lens systems taught in the above-mentioned Japanese Unexamined Patent Publication Nos. 2002-48975, 2005-37727, 2006-276897 and 2007-286577 (the negative-lead type zoom lens system), since the zoom ratio thereof is approximately 3, the zooming capability is insufficient. Moreover, if such types of zoom lens systems are used to achieve a zoom ratio of approximately 4.0, the size of the lens elements thereof becomes too large; and, if an attempt is made to miniaturize such lens systems while attaining the same optical quality, the number of lens elements has to be increased, and many glass aspherical lens elements have to be used, which undesirably increases production costs. For example, in the zoom lens systems disclosed in the above-mentioned Japanese Unexamined Patent Publication Nos. 2005-099091, 2005-084647, 2005-084648 and 2005-084649, since a large number of glass aspherical lens elements are used, the production costs thereof are high, and miniaturization of the zoom lens systems is insufficient.
On the other hand, in the positive-lead type zoom lens system having four lens groups, the overall length thereof when in use can be shortened even if the zoom ratio is increased. However, since the number of lens groups increases, it is difficult to reduce the overall length of the zoom lens system when retracted to the accommodated position, and the diameter of the first lens group becomes large. Consequently, miniaturization of the positive-lead type zoom lens system is insufficient.
Accordingly, a further miniaturized negative-lead type zoom lens system of three lens groups, which has a zoom ratio of approximately 4.0 and can be produced at low costs, is desired.